The Mont-Saint-Michel Bay (French: baie du Mont-Saint-Michel, pronounced [bɛ dy mɔ̃ sɛ̃ miʃɛl]; Breton: Bae Menez-Mikael) is located between Brittany (to the southwest) and the Normandy peninsula of Cotentin (to the south and east). In 1979, the bay was named a UNESCO World Heritage Site because of its beauty and its role in the Christian tradition. The area has large tides, more than 10 meters high, which expose much of the bay when the water level is low. Two granitic islands are found in the bay: Tombelaine and Mont-Saint-Michel. Many birds and harbor seals live in this region.

General considerations

Mont-Saint-Michel Bay covers an area of about 500 kilometers (190 square miles). The following towns and villages surround the bay, listed from north to south:

• In Normandy: Carolles, Champeaux, Saint-Jean-le-Thomas, Dragey-Ronthon, Genêts, Vains, Marcey-les-Grèves, Avranches, Le Val-Saint-Père, Céaux, Courtils, Huisnes-sur-Mer, Pontorson, Le Mont-Saint-Michel, Beauvoir.
• In Brittany: Saint-Georges-de-Gréhaigne, Roz-sur-Couesnon, Saint-Broladre, Cherrueix, Mont-Dol, Le Vivier-sur-Mer, Hirel, Saint-Benoît-des-Ondes, Saint-Méloir-des-Ondes, and Cancale.

Geography

Three small rivers flow into the bay and cross it during low tide: the Couesnon, which is blocked by a causeway to the west of Mont Saint-Michel, the Sée, and the Sélune. The very gentle slope of the bay and the large tides create a tidal wave called a mascaret in these rivers, which can move upstream for many kilometers.

Three larger rivers also flow into the bay: the Sélune, the Sée, and the Couesnon. According to some local stories, the Couesnon was once the boundary between Normandy and Brittany but was later moved west of Mont Saint-Michel, placing it in Normandy. In reality, the boundary is not at the river but 4 kilometers (2.5 miles) farther west, at the foot of Mont Saint-Broladre.

Mont-Saint-Michel Bay experiences some of Europe’s largest tides. On average, the tides have an amplitude of 10 meters (33 feet), reaching up to 12 meters (39 feet) and sometimes 16 meters (52 feet) during the highest tides. This happens partly because the bay is very shallow and the Cotentin Peninsula blocks the ocean’s movement.

The ocean moves quickly during both high and low tides, reaching as far as 10 kilometers (6.2 miles) inland. Victor Hugo once described the tides as moving "as swiftly as a galloping horse." However, the full extent of the tides is only visible at the bay’s entrance, where the sea level around Mont Saint-Michel is higher than the low tide level. The actual speed of the tides is closer to a walking human, with the fastest recorded speed being about 6.1 kilometers per hour.

The tides mix the water, creating a rich and diverse ecosystem. The intertidal zone—the area of the coast affected by tides—is about 200 square kilometers wide.

During the highest tides, a tidal bore can be seen, though this is less common now due to restoration work in the bay.

Levee and polders

Polders in Mont-Saint-Michel Bay were created as early as the 8th century. Between "Pointe de Château-Richeux" and Mont Saint-Broladre, seashell deposits were used to build levees around Dol marsh. To the east of Sainte-Anne chapel is the "Duchesse-Anne" levee. This levee is 20 kilometers (12 miles) long and was made during the 11th century using granite rocks. It marks the southern edge of the polders.

In 1856, M. Mosselman, who started the "Cie des Polders de l'Ouest," was allowed to farm 3,800 hectares of land along the shoreline between Sainte-Anne chapel, Moidrey bay, Roche-Torin, and Mont Saint Michel.